25mm armoured to 16mm earth conductor

[Mike Blair]

i have to send a 75meter run of a 3 core 25mm armoured from the meter to the fuse board. now due to the lenth i have to have a switched fuse and possibly a isolator on the otherside. due to the earth being 25mm i know alot of isolators only take 16mm.

is the use of a helely block perfered to down size the cable one each side?

 

[Leesparkykent]

25mm 2core and separate 16mm green and yellow, 25mm 2core and use armour as CPC if bonding permits or 25mm 3core and use a reducing pin to 16mm if 25mm wont fit in to isolator EMT. Un-insulated Solid Copper Pin - SWA - http://www.swaonline.co.uk/cable-terminations/copper-pin-terminals/uninsulated-solid-copper-pin

 

[Mike Blair]

got too bury the cable 75meters in soil would the earth be aloud to be separate or would it need mechanical protection?

 

[Leesparkykent]

got too bury the cable 75meters in soil would the earth be aloud to be separate or would it need mechanical protection?

16mm is the minimum CSA permitted being protected against corrosion by a sheath and not protected against mechanical damage. Table 54.1 page 163 BYB.

 

[Mike Blair]

do seperate 16mm earthing conductor is aloud to be burryed?

 

[Leesparkykent]

do seperate 16mm earthing conductor is aloud to be burryed?

yes, I have posted the relevant section of the regs above.

 

[Midwest]

Think you'll find the difference in price between 25 & 16mm swa, is actually cheaper per metre than 6491x, and less hassle installing just one cable. Lees suggestion with the solid copper pin, negates any issues in isolator. Long run mind?

PS, edit not cheaper, looking at the wrong table! But certainly less hassle!

 

[Wilko]

Hi Lee and MW, just a dumb question but isn't it ok to use SWA armour as both CPC and main bonding conductor? I haven't done a long run like this and just trying to understand it. May be the Zs is too high without the additional 16 or 25mm conductor, or prohibited by another reg? Cheers, David

 

[Leesparkykent]

Hi Lee and MW, just a dumb question but isn't it ok to use SWA armour as both CPC and main bonding conductor? I haven't done a long run like this and just trying to understand it. May be the Zs is too high without the additional 16 or 25mm conductor, or prohibited by another reg? Cheers, David

Hi mate if using another metal for a bonding conductor other than copper which the tables are based on the other metal has to offer at least equivalent conductance. 0.004 is the simplified resistance coefficient per degree C at 20 degree C given by BS 6360 for copper and aluminium conductors. The resistivity of steel (the armour) changes with carbon content. Guidance note 8 gives a value of 8.5.

For example if 10mm copper is the minimum CSA for a bonding conductor for the installation but you was using the armour or the armour was forming part of the bonding conductor then the armour would have to have a minimum CSA of at least 85mm. 10mmx8.5=85mm

 

[Wilko]

Got it, thanks !
I looked at my GN8, found table B1 in appendix B. If I'm reading it right the 25mm SWA's armour is man enough if you use 3 core, ha ha, where you won't need it cause you've now got a nice 25mm of cu to use.

 

[Leesparkykent]

My guidance note 8 is In the office but from memory you would have to use something like 95mm 2core SWA.
25mm 3core SWA would have an CSA of something around 62mm? which is below 85mm so wouldn't be adequate for bonding but would be fine as a circuit protective conductor. Table 54.7 BS7671 is to calculate/select the cross sectional area of every protective conductor, other than a protective bonding conductor which I think table B1 is based on.....could be table B2 though.

 

[netblindpaul]

I think that 25mm would be OK for a cpc, but, not a cpc & bonding conductor.

Does it need to be a bonding conductor, and if so for TN-S or TN-C-S?

 

[westward10]

The armour of a 2 core 25.0 SWA cable is approximately 6.0 in terms of copper, Lee is quite correct in saying it would have to be at least 95.0 for a 2 core to be adequate to support the minimum size of 10.0 bonding.

 

[Livia Susan]

I think you need a bonding conductor.

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Life Saver Essays

 

[westward10]

I think you need a bonding conductor.

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Life Saver Essays

?

 

[Livia Susan]

Bonding conductor are earthing conductors, also called as earthing leads that connects the earth terminal to the earth electrode.

 

[westward10]

Bonding conductor are earthing conductors, also called as earthing leads that connects the earth terminal to the earth electrode.

See Definitions in BS7671.

 

[Leesparkykent]

must be American?

 

[westward10]

must be American?

You are correct just checked her profile.